To measure the relative position of two machine parts, a scale is attached to one of the machine parts, and a scanning unit is attached to the other of the machine parts movable relative to each other. During the position measuring, a graduation marking of the scale is scanned by the scanning unit.
A distinction is made between two basic principles when attaching a scale to a carrier. In the case of the first basic principle, the scale is attached to the carrier such that it is able to expand freely with respect to the carrier in response to temperature changes. In this case, fastening elements that are deflectable in the measuring direction, or an elastic adhesive layer are used for the attachment.
In the case of the second basic principle, the scale is rigidly attached to the carrier. In this instance, the carrier and the scale may be made of a material having the same expansion coefficient. If the carrier and the scale are made of different materials, the thermal characteristic of the carrier is forced on the scale. In the case of the second basic principle, the fastening is accomplished via thin, rigidly curing adhesive layers or by direct contact, such as optical contacting.
For highly accurate position measuring, scales made of glass or glass ceramic having a negligible expansion coefficient are used. These scales may be effectively machined, so that direct bonding on opposing surfaces is used, as described in German Published Patent Application No. 101 53 147.
The problem in direct bonding a scale is that the connection can easily be disturbed by impurities or the formation of air bubbles. Moreover, the joining surfaces must be very even, which requires great effort. These problems are amplified in the case of relatively large-area scales. For this reason, the direct bonding of scales has not gained acceptance.